Underwater Abrasion Resistance of Fibre Reinforced-Latex Modified Concrete with Granulated Rubber

In determining the appropriate concrete composition, we relied on previous research, where we designed suitable concrete compositions to meet the requirements to be installed in the spillways of the dam structures on the Lower Sava River, considering the following findings:

Concrete compositions were prepared by using Portland cement with a minimum 80% proportion of clinker and a mixed addition of limestone and slag, type: CEM II/A-M(LL-S) 42.5 R, which is in accordance with the SIST EN 197–19 standard. The aggregate was obtained by separation of the natural crushed gravel from the alluvial quaternary deposit of the Sava River on the site. Fractions 0–4, 4–8, and 8–16 mm were used. Four samples of different concrete composition were intended for test purposes (Table 1): The ABR-1 composition is adopted as control composition, which is basically the same as the composition of abrasion resistant concrete built in the spillways of the first hydropower plant in the cascade on the Lower Sava River. In ABR-2 composition and all further modifications the nominal maximum gravel of 8 mm was adopted. The ABR-2 composite with smaller modifications was used in concretes on the spillway of the second hydropower plant in the cascade on the Lower Sava River.

In the following compositions, the proportion of fine fraction mineral aggregate (0–4 mm) was partly replaced by rubber aggregate. The rubber aggregate used in this study is the end product of recycling scrap for vehicle tyres with a characteristic cubic grain shape, similar to the usual crushed mineral aggregate. Rubber aggregate of different fractions (from 0 to 3,5 mm), mostly of uniform composition, were used in the study. With the ABR-3 composition the mineral additive (SiO₂ > 90%) was replaced by polymeric binder (styrene-butadiene copolymer latex with dry portion in dispersion 48%); the proportion of steel fibres was replaced by doubling polypropylene fibres (L = 10 mm, Ø30 ~ 40 μm); the proportion of the finest fraction (0–4 mm) was partially replaced by rubber aggregate. The ABR-4 composition represents a minor modification of the ABR-3 composition, in which steel fibres were added in a doubled amount compared to the control composition. The value of the w/c ratio in the composites did not vary considerably.

The mixtures of concretes were prepared in the laboratory mixer with a vertical shaft and with a volume of 75 dm³. Right after the mixing the fresh concrete properties, such as: temperature (SIST EN 12350–1:2019), slump (SIST EN 12350–2:2019), air content (SIST EN 12350–7:2019) and density (SIST EN 12350–6:2019) were determined, following the standard procedures (Table 2). The average values of investigation results of the hardened concrete are given in Table 3: (1) Compressive strength and density were performed at the ages of 28, 56, and 90 days (SIST EN 12390–3:2019 and 12390–7:2019), respectively, on cubicles of dimensions of 15 cm, by taking three samples of each composition; (2) The static modulus of elasticity of the concrete was defined at the ages of 28, 56, and 90 days on prisms of 10/10/40 cm, taking three samples per each composition (DIN 1048, Part 5); (3) Abrasive resistance test was performed at the ages of 90 and 180 days, on cylinders of Ø30/10 cm, taking one sample per composition (ASTM C 1138).

The research work was performed in accordance with standard ASTM C 1138 method for the reason that a number of comparisons and results exist in the literature on the basis of which the results of our research work could be evaluated. The test method can only be used to determine the relative resistance of the material to the abrasion action of waterborne particles. The standard procedure of the investigation provides for the measurement of the specimen surface wear at 12-h intervals, while the total investigation time is 72 h. The result of the test is the average depth of wear expressed by the average wear volume on the surface of the specimen for the duration of the test (Liu 1981).

It can be seen from the Figs. 1 and 2 that all concretes achieve adequate abrasion resistance, while concretes with the addition of rubber aggregate show a significantly higher abrasion resistance than conventional compositions. A comparison between compositions with rubber aggregate show an improvement in the abrasion resistance of the ABR-4 composition with the addition of steel fibres. The abrasion resistance of the conventional compositions has not increased with age, moreover it has even slightly decreased in the case of ABR-2 composition. However, generally we can confirm that also the concrete samples with the addition of rubber aggregate show an improved resistance towards abrasion with the increasing age of the sample.

The dynamics of wear progression was also analysed, where the test sample ABR-3 at 90 days of age have the highest initial wear increment, while all other compositions show similar initial wear increment and ABR-4 having the lowest one (Fig. 3). As expected, in the case of compositions with a rubber aggregate after 24 h of test time a decrease in the dynamic of wear progression is detected and the wear progression remains the same until the end of the test, while on the contrary, with conventional concrete compositions, the wear progression increased slightly throughout the duration of the test.

At the age of 180 days the initial wear increment after 12 h of testing is lower for all compositions than for those at the age of 90 days (Figs. 3 and 4). For rubber aggregate compositions the wear progression decreases with the duration of the test and is lower than for the cases at the age of 90 days The ABR-4 composition shows a very uniform wear progression throughout the test and a lower one compared to the ABR-3 composition. However, the conventional compositions show a different wear dynamics pattern to that observed at 90 days. After an initial wear increment a slight decrease is observed after 24 h of testing after which the wear starts to continuously increase. The more pronounced is the wear dynamics of the ABR-2 composition, which is consistently higher compared to others, while the wear progression of the ABR-1 composition remains of comparable magnitude.

Given that the previous field measurements showed a very good agreement between the field wear results and the ASTM C1138 abrasion resistance results, we decided to repeat the abrasion resistance studies on a large-scale field model in natural conditions. The opportunity to set up a field model was demonstrated by the restoration work on a small torrent in the highlands. As part of the remediation works, it is planned to establish extensive monitoring on the watercourse, in the scope of monitoring hydrological-hydraulic parameters and sediment transport along the watercourse. The field model is designed as a trough spillway in a watercourse bed with reinforced banks that direct the water flow to the spillway chute, where the test plots are placed at the bottom of the chute.

In 2022, we placed 8 test plots, 0.5/0.5 m in size, 10 cm thick, at a distance of 0.5 m from each other on the chute of the spillway. The test plots were prepared at the same time as the test specimens for the laboratory tests, 2 test plots for each composition. The test plots were prepared by pouring the concrete in special wooden moulds, using a vibration pin, and afterwards they were manually finished. Then the moulds with test plots were kept covered with plastic foil in controlled climate conditions at 20 ℃. After one day the test plots were taken out of the moulds and kept in controlled climate condition until installation in the field. Prior to installation in the field, the test plots were measured under laboratory conditions using a photogrammetric method with a data capture accuracy between 0.3 and 0.4 mm. The test plots were framed with 2.5 cm thick wooden slats before installation and the bottom was covered with a foil to prevent the test plot from sticking to the concrete base and to allow the test plot to be removed after the investigation is completed. A network of geodetic points is embedded in the concrete base of the spillway chute to allow for periodic geodetic surveys to be carried out during the duration of the investigation, which is expected to last at least 2 years (Fig. 5). After the investigation is completed, the test plots will be removed from the field model and a final geodetic survey will be carried out under laboratory conditions, which will also serve as a reference for the wear rate in natural condition. After the completion of field measurements, we will carry out an abrasion resistance test according to the ASTM C1138 method and perform comparisons between the results of laboratory measurements in the entire duration of the investigation and measurements in test plots. The purpose of this comparison is to confirm the suitability of the ASTM C1138 method for predicting the development of abrasion resistance of concrete on water structures.